Developments in certain techniques make it necessary to have available cast irons, or more generally materials, that are capable, in particular, of retaining their mechanical qualities and their qualities of resistance to oxidation at higher and higher temperatures, in particular temperatures greater than 900.degree. C.
This applies in particular to the automobile industry where the increase in performance of vehicle engines gives rise to increasingly severe conditions, and in particular temperature conditions, that the components of the engines must be capable of withstanding. In particular, certain parts of engines such as exhaust manifolds and turbine housings are subjected to ever increasing thermal and mechanical stresses, be they maximum temperatures, temperature gradients, thermal shocks, mechanical stresses, creep when hot, or thermal fatigue.
At present, the cast irons available for such temperature ranges are austenitic cast irons having a high nickel content. Typically the nickel content lies in the range 20% to 35% by weight for temperatures greater than 900.degree. C. For temperatures greater than 1000.degree. C., it is also necessary to add silicon. The drawback with such cast irons is that they make use of large quantities of nickel. Nickel has the drawback both of being expensive and also of being considered as a strategic material, and thus of suffering from very large fluctuations in price.
In addition, it is known that in motor manufacture, economic constraints relating to competition are becoming more and more acute and it is therefore particularly advantageous to be able to use materials of low cost while nevertheless capable of satisfying severe conditions of use.
An object of the present invention is to provide a cast iron having properties of mechanical strength and of resistance to oxidation that are at least as good as those of known cast irons for high temperature (typically greater than 900.degree. C.), but that have manufacturing costs which are lower than known spheroidal graphite cast irons having a high nickel content.
In the present specification, the term "cast iron" should be understood as designating an alloy containing at least 85% iron.